1. Field of the Invention
The present invention relates to an optical disk on which digital data is recorded. The present invention particularly relates to an optical disk on which digital data is recorded with high density by using a pit edge multi-value recording system and an optical disk reproducing apparatus for decoding the digital data from the optical disk to reproduce it.
2. Description of Related Art
Nowadays, as a system for recording and reproducing digital data on and from an optical disk with high density, there has been developed a pit edge multi-value recording system for recording digital data by modulating positions of pit edges on the optical disk into multiple steps. In this pit edge multi-value recording system, positions of the pit edges are changed in multi-steps according to digital data to record multi-value digital data as positional information of the pit edges. By radiating an optical beam between, for example, two adjoining recording tracks, an RPR (Radial Direction Partial Response) reproducing system reproduces a plurality of pit edges at the same time on the basis of a reflected light from the optical disk. By employing the pit edge multi-value recording system and the RPR reproducing system, the digital data can be recorded on the optical disk with higher density. For example, these systems are disclosed in Japanese Laid-Open Patent Application No. 10-74322.
Generally, digital data of two values is recorded on an optical disk such as an existing CD and DVD, and RLL (Run Length Limited) code is used which limits the maximum value and the minimum value of a run length indicating the length of the digital data of the same sign. In the case of DVD, an {fraction (8/16)} modulation system, which is a kind of RLL code, is employed. In this {fraction (8/16)} modulating system, with respect to the channel clock period T, the run length of the modulation code is limited to the range from 3T to 11T. A run length 14T, which exceeds this range, may be defined as a synchronous code. Further, since the run length is limited, it is possible to reproduce a clock by using a pulse edge of the reproduced two-value pulse as a phase reference. Further, the {fraction (8/16)} modulation system is a kind of DC-free code in which digital data sequence has no direct current component, so that recorded codes can be restored even if direct current component is removed from an analog reproduction signal.
On the contrary, in the above mentioned pit edge multi-value recording system, since RLL code which is effective for multi-value digital data is not known, the run length cannot be limited, and hence synchronous code having the maximum length cannot be defined. In addition, since the run length is not limited, there is a problem that it is difficult to reproduce clock by using pulse edges of multi-value pulses as phase reference. Still further, since DC-free code which is effective for multi-value digital data is not known, recorded code cannot be restored if direct current component is removed from analog reproduction signal.
The present invention has been made taking the foregoing problems into consideration, and an object of which is to provide an optical disk and an optical disk reproducing apparatus which are capable of securely detecting synchronization, stably reproducing clock and reproducing more accurate digital data at the time of reproducing an optical disk on which digital data is stored by a pit edge multi-value recording system.
According to one aspect of the present invention, there is provided an optical disk on which digital data is recorded by a pit edge multi-value recording system, including: a plurality of data areas formed with a plurality of data pits with a predetermined pit period, each of the plurality of data pits including one of a plurality of pit edge positions each corresponding to the digital data; and a plurality of servo areas formed with servo reference pits used for servo control, the data areas and the servo areas being formed alternately with each other in a circumferential direction of the optical disk, the servo area including synchronization pits functioning as synchronization reference in reproducing the optical disk, wherein a pit period 2L of the data pit and a pit length P of the synchronization pit satisfy the following equation:
P=Nxc3x97L (N is an integer not less than 2).
When this optical disk is reproduced, amplitude of a reproduction signal in a segment corresponding to the synchronization pit may be sufficiently long, and hence the synchronization detection can be performed by detecting the amplitude of the reproduction signal.
The data pits may be arranged such that the data pits formed on two neighboring tracks are in reverse phase with each other, and the synchronization pits may be arranged such that the synchronization pits formed on two neighboring recording tracks are in alignment with each other in a radial direction of the optical disk. By this, if the center line of two neighboring tracks is traced by a light beam, the amplitude of the reproduction signal is larger in the synchronization pit segment than that in the data pit segment. Thus, the synchronization pits may be readily detected, and the synchronization detection may be performed more reliably.
According to another aspect of the present invention, there is provided an optical disk on which digital data is recorded by a pit edge multi-value recording system, including: a plurality of data areas formed with a plurality of data pits with a predetermined pit period, each of the plurality of data pits including one of a plurality of pit edge positions each corresponding to the digital data; and a plurality of servo areas formed with servo reference pits used for servo control, the data areas and the servo areas being formed alternately with each other in a circumferential direction of the optical disk, the servo area including synchronization spaces functioning as synchronization reference in reproducing the optical disk, wherein a pit period 2L of the data pit and a length S of the synchronization space satisfy the following equation:
S=Nxc3x97L (N is an integer not less than 2).
When this optical disk is reproduced, amplitude of a reproduction signal in a segment corresponding to the synchronous space may be sufficiently long, and hence the synchronization detection can be performed by detecting the amplitude of the reproduction signal.
The data pits may be arranged such that the data pits formed on two neighboring tracks are in reverse phase with each other, and the synchronization spaces may be arranged such that the synchronization spaces formed on two neighboring recording tracks are in alignment with each other in a radial direction of the optical disk. By this, if the center line of two neighboring tracks is traced by a light beam, the amplitude of the reproduction signal is larger in the synchronization pit segment than that in the data space segment. Thus, the synchronization pits may be readily detected, and the synchronization detection may be performed more reliably.
According to still another aspect of the present invention, there is provided an optical disk reproducing apparatus for reproducing an optical disk described above, including: a reading unit for irradiating a laser beam onto the optical disk to generate a reproduction signal; and a synchronization detecting unit for detecting segments, in which the reproduction signal has an amplitude larger than a predetermined value, as a segment corresponding to the synchronization pit or space and for outputting a synchronization detection signal, wherein the synchronization detection signal is used as a synchronization reference in reproducing the optical disk.
In accordance with the apparatus, the reading unit irradiates the laser beam to produce the reproduction signal. Since the amplitude of the reproduction signal at the synchronization pits or spaces exceeds the predetermined value, the synchronization signal may be detected at that timing. Therefore, by determining the synchronization reference in reproduction based on this synchronization detection signal, the reproduction operation may be reliably achieved.
According to another aspect of the present invention, there is provided an optical disk reproducing apparatus for reproducing an optical disk described above, including: a reading unit for irradiating a laser beam onto a center line of two neighboring recording tracks on the optical disk to generate a reproduction signal; and a synchronization detecting unit for detecting segments, in which the reproduction signal has a peak level larger than a predetermined threshold value, as a segment corresponding to the synchronization pit or space and for outputting a synchronization detection signal, wherein the synchronization detection signal is used as a synchronization reference in reproducing the optical disk.
In accordance with the apparatus, the reading unit irradiates the light beam to trace the center line of two recording tracks to generate the reproduction signal. The synchronization detection unit outputs the synchronization detection signal. Since the synchronization pits are aligned in the radial direction of the disk, the peak level of the reproduction signal is stable. Since the data pits are formed alternately in the radial direction of the disk, the amplitude of the reproduction signal becomes smaller, and the detection of the synchronization pits or spaces may become easier. Therefore, by determining the synchronization reference based on the synchronization detection signal, the reproduction operation may be reliably synchronized.
The apparatus may further include: an A/D converter for sampling the reproduction signal in synchronism with reproduction clock to produce sample value sequence and for outputting the sample value sequence; and a clock reproduction unit for generating reproduction clock based on the sample value sequence. Thus, the reproduction signal may be sampled at correct timings, and the digital data may be accurately reproduced.
The clock reproduction unit may control frequency of the reproduction clock based on a period of the synchronization detection signal. Thus, the frequency of the reproduction clock may be easily maintained, and the stable reproduction clock may be supplied.
The clock reproduction unit may control phase of the reproduction clock based on plural sample values corresponding to positions of the synchronization pits. Thus, the phase of the reproduction clock may be maintained stably to correspond to the position of the synchronization pits or spaces.
The apparatus may further include a direct current level corrector for correcting direct current level of the sample value sequence based on the plural sample values corresponding to the positions of the synchronization pits. Thus, even if the direct current component is removed from the read signal, the direct current may be restored in the sample value sequence, and hence the digital data may be correctly reproduced.